There has been considerable interest in recent years in developing high energy density batteries with lithium-containing anodes. Lithium metal is particularly attractive as the anode active material of electrochemical cells because of its light weight and high energy density, as compared, for example, to anode active materials such as lithium intercalated carbon anodes, where the presence of non-electroactive materials increases the weight and volume of the anode, thereby reducing the energy density of the anode. The use of lithium metal anodes, or those comprising lithium metal, provides an opportunity to construct cells that are lighter in weight and have a higher energy density than cells such as lithium-ion, nickel metal hydride or nickel-cadmium cells. These features are highly desirable for batteries in portable electronic devices such as cellular telephones and laptop computers, as noted, for example, by Linden in Handbook of Batteries, 1995, 2nd Edition, chapter 14, pp. 75-76, and chapter 36, p. 2, McGraw-Hill, New York, and in U.S. Pat. No. 6,406,815 to Sandberg et al., the respective disclosures of which are incorporated herein by reference.
Thin film battery design is particularly suitable for portable electronic devices in that it brings light weight, and the high surface area allows high rate capability as well as reduced current density on charging. Several types of cathode materials for the manufacture of thin-film lithium batteries are known, and include cathode materials comprising sulfur-sulfur bonds, wherein high energy capacity and rechargeability are achieved from the electrochemical cleavage (via reduction) and reformation (via oxidation) of sulfur-sulfur bonds. Sulfur containing cathode materials, having sulfur-sulfur bonds, for use in electrochemical cells having lithium or sodium anodes include elemental sulfur, organo-sulfur, or carbon-sulfur compositions.
For rechargeable lithium/sulfur (Li/S) cells there is a need for further enhancement of cell performance. Ideally cells should have high utilization at practical discharge rates over many cycles. Complete discharge of a cell over time periods ranging from 20 minutes (3C) to 3 hours (C/3) is typically considered a practical discharge rate. A number of approaches have been explored for the improvement of the performance and properties, such as utilization, self-discharge, charge-discharge efficiency, and overcharge protection.
Lithium anodes in nonaqueous electrochemical cells develop surface films from reaction with cell components including solvents of the electrolyte system and materials dissolved in the solvents, such as, for example, electrolyte salts and materials that enter the electrolyte from the cathode. Materials entering the electrolyte from the cathode may include components of the cathode formulations and reduction products of the cathode formed upon cell discharge. In electrochemical cells with cathodes comprising sulfur-containing materials reduction products may include sulfides and polysulfides. The composition and properties of surface films on lithium electrodes have been extensively studied, and some of these studies have been summarized by Aurbach in Nonaqueous Electrochemistry, Chapter 6, pages 289-366, Marcel Dekker, New York, 1999. The surface films have been termed solid electrolyte interface (SEI) by Peled, in J. Electrochem. Soc., 1979, vol. 126, pages 2047-2051.
The SEI may have desirable or undesirable effects on the functioning of an electrochemical cell, depending upon the composition of the SEI. Desirable properties of an SEI in an electrochemical cell comprising a lithium anode include being conductive to lithium ions and at the same time preventing or minimizing lithium consuming reactions, such as those with electrolyte salts, electrolyte solvents, or soluble cathode reduction (discharge) products. Undesirable properties of the SEI may include reduced discharge voltage and reduced capacity of the cell. Soluble cathode reduction products from sulfur-containing cathode materials are known to be very reactive toward lithium anodes indicating that any SEI formed in Li/S cells is typically ineffective in preventing or minimizing lithium consuming reactions (these reactions are often termed lithium corrosion).
Approaches to protect lithium in Li/S cells have been described by Visco et al. in U.S. Pat. No. 6,025,094; by Nimon et al. in U.S. Pat. Nos. 6,017,651 and 6,225,002; and by Skotheim et al. in U.S. patent application Ser. Nos. 09/721,578 and 09/864,890.
Sulfur utilization in Li/S cells is dependent on a number of factors, including among others, formulation of the cathode, discharge rate, temperature, and electrolyte composition. As used herein, “a 100% utilization” (also called “sulfur utilization”) assumes that if all elemental sulfur in an electrode is fully utilized, the electrode will produce 1675 mAh per gram of sulfur initially present in the electrode. Among the prior art references that discuss and teach sulfur utilization are the following:
(1) U.S. Pat. No. 4,410,609 Peled et al. claimed to have achieved sulfur utilization of about 90% in Li/S cells employing THF or THF/toluene electrolyte solvents, but only at very low discharge rates (two months for a single discharge).
(2) Peled et al. in J. Electrochem. Soc., 1989, vol. 136, pp. 1621-1625 found that in dioxolane solvent mixtures similar Li/S cells achieve a sulfur utilization of no more than 50% at discharge rates of 0.1 mA/cm2 and 0.01 mA/cm2.                a. (3) Chu in U.S. Pat. No. 5,686,201 describes a Li/S cell with a polymeric electrolyte that delivers 54% utilization at 30° C. and a low discharge rate of 0.02 mA/cm2. At 90° C. a utilization of 90% at a discharge rate of 0.1 mA/cm2 was achieved.        
(4) Chu et al. in U.S. Pat. No. 6,030,720 describe liquid electrolyte Li/S rechargeable cells with sulfur utilization of approximately 40% for more than 70 cycles at discharge rates of 0.09 mA/cm2 (90 μA/cm2) and 0.5 mA/cm2 (500 μA/cm2). Another example (Example 4) describes a sulfur utilization of 60% over more than 35 cycles but at the low discharge rate of 0.09 mA/cm2.
(5) Cheon et al. in J. Electrochem. Soc., 2003, vol. 150, pp. A800-A805, describe various properties including rate capability and cycle characteristics of rechargeable Li/S cells. In FIG. 5 are shown charge and discharge profiles for Li/S cells, with 0.5 M lithium triflate in tetraglyme as electrolyte, from which charge-discharge efficiencies can be estimated. A charge-discharge efficiency at the 30th cycle of approximately 67% is estimated for cells charged at 0.26 mA/cm2 and discharged at 0.26 mA/cm2 and an efficiency of approximately 48% for cells charged at 0.26 mA/cm2 and discharged at 1.74 mA/cm2. The sulfur utilization of these same cells is shown to be 37% and 28%, respectively, at the 30th cycle.
Many lithium-based electrochemical cells, including Li/S cells, may be recharged by applying external current to the cell. The general mechanism for recharging many lithium rechargeable cells is described by Hossain in Handbook of Batteries, 1995, 2nd Edition, chapter 36, p. 1-28, McGraw-Hill, New York, and for Li/S cells by Mikhaylik et al. in J. Electrochem. Soc., 2003, vol. 150, pp. A306-A311. When a cell is recharged it may be unintentionally overcharged, which could lead to various undesirable reactions such as destruction of the cell electrolyte, corrosion of the current collectors, degradation of the cell separators, and irreversible damage to the positive or negative electrode. Overcharge protection has been provided in lithium cells by the use of redox shuttle additives, as described, for example, by Narayanan et al. in J. Electrochem. Soc., 1991, vol. 138, pp. 2224-2229, Golovin et al. in J. Electrochem. Soc., 1992, vol. 139, pp. 5-10, and Richardson et al. in J. Electrochem. Soc., 1996, vol. 143, pp. 3992-3996. The redox shuttle additive species is oxidized at the cathode during overcharge, diffuses to the anode electrode, where it is reduced to the original species and diffuses back.
In Li/S cells an intrinsic redox shuttle is known that provides overcharge tolerance or protection, as described, for example, in U.S. Pat. No. 5,686,201 to Chu. Chu et al. in Proceedings of the 12th Annual Battery Conference on Applications & Advances, 1997, pp. 133-134, state that the shuttle in Li/S cells limits overcharging, and provide examples of cell voltage holding constant during extended overcharge as illustrated in FIG. 4 on page 134.
U.S. Pat. No. 5,882,812 to Visco et al. describes a method of protection of rechargeable electrochemical energy conversion devices against damage from overcharge. Specifically, such devices may be characterized as including the following elements: (1) a negative electrode; (2) a positive electrode containing one or more intermediate species that are oxidized to one or more oxidized species during overcharge; and (3) a tuning species that adjusts the rate at which the oxidized species are reduced, thereby adjusting the voltage at which overcharge protection is provided. The oxidized species produced during overcharge move to the negative electrode where they are reduced back to the intermediate species as in a normal redox shuttle. The overcharge protection systems are described as applicable to many different cells, particularly those with alkali metal negative electrodes, including lithium/organosulfur cells, lithium/(inorganic sulfur) cells, lithium/(metal oxide) cells, lithium/(metal sulfide) cells, and carbon anode cells. The tuning species described include organosulfur compounds, and surface active agents including: organoborates such as trimethylborate, boroxines, such as trimethylboroxine; phosphorus containing compounds including polyphosphazenes and phosphates such as Li3PO4; carbonates such as Li2CO3; nitrogen containing compounds including nitrates such as LiNO3; and organonitrogen compounds such as phenylhydrazine.
Gan et al. in U.S. Pat. Nos. 6,136,477 and 6,210,839 describe nitrates and nitrites as additives for electrolytes in lithium ion cells to reduce 1st cycle irreversible capacity. In U.S. Pat. No. 6,060,184 Gan et al. describe nitrate additives for nonaqueous electrolytes that provide increased discharge voltage and reduced voltage delay in current pulse discharge, for example in alkali metal cells with SVO (silver-vanadium oxide) positive electrodes.
Redox shuttles in electrochemical cells, however, have also been shown to have an undesirable impact on cell properties, such as leading to self-discharge. Rao et al. in J. Electrochem. Soc., 1981, vol. 128, pp. 942-945, the disclosure of which is incorporated herein by reference, describe the self discharge of Li/TiS2 cells due to the presence of elemental sulfur impurities, which act through a redox shuttle mechanism. The sulfur impurities become part of a polysulfide shuttle. Sulfide ions or low chain polysulfides are oxidized at the cathode to higher polysulfides that are soluble in the electrolyte. These higher polysulfides diffuse through the electrolyte to the anode where they are reduced to lower polysulfides that in turn diffuse back through the electrolyte to the cathode to be reoxidized to higher polysulfides. This redox shuttle causes a continuous current flow in the cell, resulting in a depletion of the cell's stored capacity. This phenomenon is called self-discharge. In U.S. Pat. No. 4,816,358 Holleck et al. describe a method of reducing self-discharge in lithium cells, such as Li/TiS2 cells, which comprise cathodes containing sulfur as an impurity. The method uses scavengers, for example, metals or metal ions, that react with sulfur impurities to form stable sulfides thus reducing self discharge.
For rechargeable batteries, determining the point at which to terminate charging is important for efficient charging, longevity of the battery, and for safety. A number of methods are known for charging batteries and for determining the point of termination of the charge. U.S. Pat. No. 5,900,718 to Tsenter and U.S. Pat. No. 5,352,967 to Nutz et al. summarize some of these charging and charge termination methods particularly useful for nickel batteries, such as nickel-cadmium, nickel-hydrogen and nickel metal-hydride. Prominent among the termination methods are delta temperature/delta time (dT/dt), delta voltage/delta time (dV/dt), and termination at a predetermined voltage.